1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine, and in particular, to one for an automotive vehicle such as a so-called flexible fuel vehicle (FFV).
2. Description of the Related Art
There is a motor vehicle in the form of a so-called flexible fuel vehicle (FFV) which is able to travel also with a mixed fuel of various compositions of alcohol and gasoline, other than with gasoline alone.
Alcohol has the content of C (carbon) atoms differing with respect to that of ordinary gasoline (blended or mixed fuel), so when a blended fuel of alcohol and gasoline is supplied to an internal combustion engine used for the flexible fuel vehicle, it is necessary to adjust the amount of fuel to be injected according to the alcohol concentration in fuel.
In such a flexible fuel vehicle, there has hitherto been known one in which an estimation of the alcohol concentration in fuel is performed by using a correlation between the alcohol concentration and an air fuel ratio feedback correction coefficient that is calculated based on an air fuel ratio in an exhaust gas detected by an air fuel ratio sensor (see, for example, a first patent document: Japanese patent application laid-open No. 2004-245097).
There will be a delay until fuel in a fuel tank, of which the alcohol concentration has been changed due to refueling, arrives at an injector that supplies the fuel to an internal combustion engine, so the alcohol concentration of the fuel to be supplied from the injector changes with the delay. In addition, this delay becomes larger in a returnless fuel supply system.
According to the above-mentioned prior art, however, the alcohol concentration is calculated in real time from the air fuel ratio feedback correction coefficient, so it is possible to detect the change in the alcohol concentration of the fuel supplied from the injector without delay.
On the other hand, the delay of the change in the alcohol concentration until the fuel in the fuel tank arrives at the injector that supplies the fuel to the internal combustion engine is a prescribed delay that can be beforehand designed or calculated on the desk from the capacity of a fuel supply pipe, the capacity of an injector fuel supply tank, the flow rate of fuel in the fuel supply system, and so on.
In addition, the air fuel ratio feedback correction coefficient includes not only the influence of the change of the alcohol concentration but also the influence of the change of the air fuel ratio due to the introduction of a vaporized gas or due to the characteristic variation of fuel supply devices such as injectors, an air flow meter, etc. These influences are detected in a mixed manner, and in order not to cause false detection, the introduction of the vaporized gas is inhibited during detection of the alcohol concentration thereby to remove its resultant influence. Moreover, the detection of the characteristic variation of the fuel supply devices is also inhibited, so that the detection accuracy of the alcohol concentration can be improved.
According to the conventional methods, however, there has been a problem that it is necessary to estimate the alcohol concentration at all times during the time when the alcohol concentration of fuel to be supplied from an injector is changing, and trouble occurs if the estimation of the alcohol concentration is interrupted on the way thereof. For example, for a certain period of time after the engine is stopped and then restarted while the alcohol concentration is changing, the air fuel ratio sensor does not become activated, so the air fuel ratio feedback is stopped and the estimation of the alcohol concentration is also stopped. Therefore, the change in the alcohol concentration can not be temporarily detected, and variation in the air fuel ratio will occur. In addition, as the temperature of engine cooling water becomes low, the time until the air fuel ratio sensor is activated will be delayed very much, so a stop period for the estimation of the alcohol concentration becomes longer, and the air fuel ratio variation becomes larger, too.
Moreover, it is necessary to always perform the estimation of the alcohol concentration during the time when the alcohol concentration of the fuel to be supplied from the injector is changing, and accuracy in the air fuel ratio control is reduced upon interruption of the alcohol concentration estimation. As a result, it has been difficult to introduce vaporized gas or to perform air fuel ratio open-loop control such as enriching the air fuel ratio while interrupting the alcohol concentration estimation.